Nafion drier

Fig. 4.7. Custom-made device for interfacing a PT module to a multi-capillary gas chromatograph using a Nafion drier for water removal. (Reproduced with permission of the Royal Society of Chemistry.)...

Possible and, for certain carrier materials, already known decomposition reactions have been mentioned. For this reason, another method is favoured by the EPA for air analysis. Electropolished and passivated canisters, called SUMMA canister expressing the highest inertness, of about 2 L capacity, maximum up to 15 L are evacuated for sample collection. The whole air samples collected onsite on opening the canister can be measured several times in the laboratory following EPA methods such as TO-14 or TO-15. Suitable samplers are used, which are connected online with GC-MS. Cryofocussing is used to concentrate the analytes from the volumes collected. If required, the sample can be dried with a semiper-meable membrane (Nafion drier) or by condensation of the water (MCS, see also section 2.1.5.2). Adsorption materials are not used in these processes. 

A cryoconcentrator with a three-way valve system is used for concentration. The sample can be applied by two routes without having to alter the screw joints on the tubing. Usually the inlet is via control of a mass flow regulator to a cryofocusing unit. The direct measurement of the sample volume provides very precise data. A Nafion drier is used to dry the air (Schnute and McMillan, 1993). 

The Nafion drier is a system for removal of water from the air sample, which uses a semi-permeable membrane (see Figure 2.211). Nonpolar compounds pass the membrane unaffected while polar ones, such as water, are held by the membrane and diffuse outwards. The outer side of the membrane is dried by a clean air stream and the water thus separated is removed from the system. The Nafion drier is recommended by the TO-14 method to prevent blockage of the cryofocusing unit by the formation of ice crystals. 

Concentrator Grasby Nutech model 3550A cryoconcentrator with 354A cryofocusing unit, Nafion drier... 

Sample injection The sample is drawn from the SUMMA sampling canister through the Nafion drier and reaches the cryoconcentrator cooled to —160°C. It is then heated rapidly to transfer the sample to the cryofocusing unit of the GC (Figure 4.2)... 

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